The present invention pertains generally to instrumentation for monitoring physical parameters of operating machines and more particularly to instrumentation for monitoring mechanical vibrations of component parts of large electrical power generators.
It is frequently desirable or necessary to determine the magnitude and/or pattern of mechanical vibrations in some part of an operating machine. For example, in some machines it may be necessary to continuously monitor vibrations in order to initiate protective action in case there is a trend toward destructive levels of vibration. In other situations, vibration information might be sought in order to carry out a test program for machine development or in order to pinpoint the source or cause of vibrations.
One application for vibration monitoring is the large electrical power generators used by the utility companies for generating power on a commercial scale. In these machines, it is particularly desirable to determine the vibrations occurring in the connection rings which electrically interconnect the phase windings and which carry the load current to bushings leading through the generator casing. These connection rings carry thousands of amperes of current at voltage levels running well into the kilovolt range. Vibrations can arise in the connection rings as a result of the intense electromagnetic forces within the generator which in turn are caused by the large alternating current flows and by the rotating magnetic field. The connection rings are mechanically restrained to the extent possible, of course, and flexible joints are used to absorb vibrations so that they are not transmitted to the output bushings of the generator.
To protect the connection rings and flexible joints from fatigue failure due to vibrations, means have long been sought which would allow vibrations to be continuously monitored in these and other structural members of the generator. Obtaining accurate vibration information from these rings, however, and indeed from other components of these machines, has been hampered by the necessity of having a vibration sensor and its associated circuitry located within the close confines of the generator wherein they are exposed to the effects of high current voltage, and intense electromagnetic fields. The conventional vibration monitoring method in which a piezoelectric accelerometer transducer, attached to the vibrating component, generates electrical charge to be converted into a conventional electronic signal by a charge converter, has not been entirely successful due to the hostile electrical environment and to the relatively tight spacings around the generator end windings. The problems, however, generally have been associated more with the charge converter than with the transducer, and with the need for isolation between the electronic circuitry and the remote (external to the generator) signal handling circuitry.
Conventional charge converters are generally of two types. The first type is a simple circuit consisting of an operational amplifier having a negative feedback loop which includes a capacitor which is responsive to be charged by the electrical charge generated by the transducer. The transducer output is connected directly to the input terminals of the operational amplifier. While this type of charge converter is small, uncomplicated, and can be fitted well into the tight spacing around generator end windings, it suffers from an inability to provide good common mode rejection, ground loop isolation, and freedom from the effects of the differences in potential arising in interconnecting cabling and so forth as a result of the severe electrical environment within the generator. These latter drawbacks particularly hamper charge converters if they are to be used in generators wherein large common mode signals and other electrical interferences are readily generated by the current and voltage produced by the machine. Under such conditions, the vibration signal is simply overwhelmed and lost if the common mode signals and interferences are not rejected. Furthermore, significant errors arise from ground loops caused by significantly different potentials appearing at common connection points relatively close together physically.
The second type of conventional charge converter partially overcomes the common mode rejection and ground loop problems by using a number of discrete components, separate isolated power supplies, and isolation amplifiers. However, the circuitry is quite complex and is not readily adaptable to use in an electric generator.
Accordingly, it is among the objects of the present invention to provide apparatus for monitoring mechanical vibrations in a component part of an operating machine wherein the apparatus is simple, economical to manufacture and install, is particularly adaptable to installation in and around the end windings of a large electrical power generator, is operable to provide a high degree of immunity to undesired common mode signal components, and other electrical interferences and is substantially free of ground loop effects.